Nowadays, spiral steel pipes are being used increasingly for line pipes for transferring crude oil and natural gas, because steel pipes having a large diameter can be efficiently manufactured using a process in which pipe-making is performed by forming a steel sheet into a spiral configuration. In particular, pipe lines for a long-distance transportation are used under increased pressure due to a requirement for an increase in transportation efficiency and often run through cold districts because many oil wells and gas wells are situated in cold districts. Therefore, such line pipes to be used are required to have increased strength and toughness. Moreover, line pipes are required to have a low yield ratio from the viewpoint of buckling resistance and earthquake resistance. The yield ratio in the longitudinal direction of a spiral steel pipe is substantially equal to that of a hot rolled steel sheet which is a raw material of the spiral steel pipe, because a yield ratio is scarcely changed under a pipe-making process. Therefore, in order to decrease the yield ratio of a line pipe manufactured using a pipe-making process for a spiral steel pipe, it is necessary to decrease the yield ratio of a hot rolled steel sheet which is a raw material of the line pipe.
In order to meet such a requirement, for example, Patent Literature 1 discloses a method for manufacturing a high tensile strength hot rolled steel sheet for a line pipe with low yield ratio excellent in terms of low-temperature toughness. It is said that the technique described in Patent Literature 1 includes heating a steel slab having a chemical composition containing, by mass %, C: 0.03% to 0.12%, Si: 0.50% or less, Mn: 1.70% or less, Al: 0.070% or less, and at least one of Nb: 0.01% to 0.05%, V: 0.01% to 0.02%, and Ti: 0.01% to 0.20% at a temperature of 1180° C. to 1300° C., performing hot rolling on the heated slab under conditions that the roughing delivery temperature is 950° C. to 1050° C. and that the finishing delivery temperature is 760° C. to 800° C., cooling the hot rolled steel sheet at a cooling rate of 5 to 20° C./s, starting air cooling for a holding time of 5 to 20 seconds on the cooled steel sheet before the temperature of the cooled steel sheet reaches 670° C., cooling the air-cooled steel sheet at a cooling rate of 20° C./s or more, and coiling the cooled steel sheet at a temperature of 500° C. or lower in order to make a hot rolled steel sheet. According to the technique disclosed in Patent Literature 1, it is said that it is possible to manufacture a high-toughness hot rolled steel sheet having a tensile strength of 60 kg/mm2 or more (590 MPa or more), a yield ratio of 85% or less, and a fracture transition temperature of −60° C. or lower.
In addition, Patent Literature 2 discloses a method for manufacturing a hot rolled steel sheet for a high strength pipe with low yield ratio. The technique described in Patent Literature 2 is a method for manufacturing a hot rolled steel sheet, the method including heating steel having a chemical composition containing C: 0.02% to 0.12%, Si: 0.1% to 1.5%, Mn: 2.0% or less, Al: 0.01% to 0.10%, and Mo+Cr: 0.1% to 1.5% at a temperature of 1000° C. to 1300° C., finishing hot rolling in a temperature range of 750° C. to 950° C., cooling the hot rolled steel sheet to a coiling temperature at a cooling rate of 10° C./s to 50° C./s, and coiling the steel sheet in a temperature range of 480° C. to 600° C. According to the technique disclosed in Patent Literature 2, it is said that it is possible, without performing rapid cooling from a temperature range in which an austenite phase is formed, to obtain a hot rolled steel sheet having a microstructure including a ferrite phase as a main phase, in terms of area fraction, 1 to 20% of a martensitic phase, a yield ratio of 85% or less, and a small decrease in yield strength after pipe-making has been performed.
In addition, Patent Literature 3 discloses a method for manufacturing an ERW pipe with low yield ratio excellent in terms of low-temperature toughness. According to the technique disclosed in Patent Literature 3, an ERW pipe is manufactured by hot rolling a slab having a chemical composition containing, by mass %, C: 0.01% to 0.09%, Si: 0.50% or less, Mn: 2.5% or less, Al: 0.01% to 0.10%, Nb: 0.005% to 0.10%, and one, two, or more of Mo: 0.5% or less, Cu: 0.5% or less, Ni: 0.5% or less, and Cr: 0.5% or less, in which Mneq, which is expressed by a relational expression regarding the contents of Mn, Si, P, Cr, Ni, and Mo, is 2.0 or more, by cooling the hot rolled steel sheet to a temperature of 500° C. to 650° C. at a cooling rate of 5° C./s or more, by coiling the cooled steel sheet, by holding the coiled steel sheet in this temperature range for 10 minutes or more, by cooling the held steel sheet to a temperature of lower than 500° C. in order to make a hot rolled steel sheet, and by performing pipe-making with the hot rolled steel sheet. According to the technique disclosed in Patent Literature 3, it is said that it is possible to manufacture an ERW pipe having a microstructure including a bainitic ferrite phase as a main phase, 3% or more of martensitic phase, and 1% or more of a retained austenite phase as needed, a fracture transition temperature of −50° C. or lower, excellent low-temperature toughness, and high plastic deformation absorption capability.
In addition, Patent Literature 4 discloses a high-toughness thick steel sheet with low yield ratio. According to the technique disclosed in Patent Literature 4, it is said that it is possible to obtain a high-toughness thick steel sheet with aow yield ratio having a mixed microstructure in which a ferrite phase having an average grain diameter of 10 to 50 μm and a bainite phase in which, in terms of area fraction, 1% to 20% of a martensite-austenite constituent is dispersed by heating a slab having a chemical composition containing C: 0.03% to 0.15%, Si: 1.0% or less, Mn: 1.0% to 2.0%, Al: 0.005% to 0.060%, Ti: 0.008% to 0.030%, N: 0.0020% to 0.010%, and O: 0.010% or less, preferably at a temperature of 950° C. to 1300° C., by performing hot rolling on the heated slab under conditions that the rolling reduction in a temperature range of (the Ar3 transformation point+100° C.) to (the Ar3 transformation point+150° C.) is 10% or more and where the finishing delivery temperature is 800° C. to 700° C., by starting accelerated cooling on the hot rolled steel sheet at a temperature within −50° C. from the finishing delivery temperature, by performing cooling with water to a temperature of 400° C. to 150° C. at an average cooling rate of 5° C./s to 50° C./s, and by performing air cooling thereafter. Here, there is no mention of the shape of a martensite-austenite constituent (rod-like or massive: described below).